The Impact of Radiation Energy on Dose Homogeneity and Organ Dose in the Göttingen Minipig Total-Body Irradiation Model

Animal models of total-body irradiation (TBI) are used to elucidate normal tissue damage and evaluate the efficacy of medical countermeasures (MCM). The accuracy of these TBI models depends on the reproducibility of the radiation dose-response relationship for lethality, which in turn is highly dependent on robust radiation physics and dosimetry. However, the precise levels of radiation each organ absorbs can change dramatically when different photon beam qualities are used, due to the interplay between their penetration and the natural variation of animal sizes and geometries. In this study, we evaluate the effect of varying the radiation energy, namely cobalt-60 (Co-60); of similar penetration to a 4-MV polyenergetic beam), 6 MV and 15 MV, in the absorbed dose delivered by TBI to individual organs of eight Göttingen minipigs of varying weights (10.3-24.1 kg) and dimensions (17.5-25 cm width). The main organs, i.e. heart, lungs, esophagus, stomach, bowels, liver, kidneys and bladder, were contoured by an experienced radiation oncologist, and the volumetric radiation dose distribution was calculated using a commercial treatment planning system commissioned and validated for Co-60, 6-MV and 15-MV teletherapy units. The dose is normalized to the intended prescription at midline in the abdomen. For each animal and each energy, the body and organ dose volume histograms (DVHs) were computed. The results show that more penetrating photon energies produce dose distributions that are systematically and consistently more homogeneous and more uniform, both within individual organs and between different organs, across all animals. Thoracic organs (lungs, heart) received higher dose than prescribed while pelvic organs (bowel, bladder) received less dose than prescribed, due to smaller and wider separations, respectively. While these trends were slightly more pronounced in the smallest animals (10.3 kg, 19 cm abdominal width) and largest animals (20 kg, ∼25 cm abdominal width), they were observed in all animals, including those in the 9-15 kg range typically used in MCM models. Some organs received an average absorbed dose representing80% of prescribed dose when Co-60 was used, whereas all organs received average doses of87% and93% when 6 and 15 MV were used, respectively. Similarly, average dose to the thoracic organs reached as high as 125% of the intended dose with Co-60, compared to 115% for 15 MV. These results indicate that Co-60 consistently produces less uniform dose distributions in the Göttingen minipig compared to 6 and 15 MV. Moreover, heterogeneity of dose distributions for Co-60 is accentuated by anatomical and geometrical variations across various animals, leading to different absorbed dose delivered to organs for different animals. This difference in absorbed radiation organ doses, likely caused by the lower penetration of Co-60 and 6 MV compared to 15 MV, could potentially lead to different biological outcomes. While the link between the dose distribution and variation of biological outcome in the Göttingen minipig has never been explicitly studied, more pronounced dose heterogeneity within and between organs treated with Co-60 teletherapy units represents an additional confounding factor which can be easily mitigated by using a more penetrating energy.